The present invention relates to a process for the production of 1,3-propanediol having a residual carbonyl content below 500 ppm, expressed as propionaldehyde, by catalytic hydrogenation of 3-hydroxypropionaldehyde in aqueous solution.
1,3-Propanediol has many potential applications as a monomer unit for polyesters and polyurethanes and as a starting material for the synthesis of cyclic compounds.
1,3-Propanediol can be produced by various processes in which the molecule is built up from a C.sub.2 and C.sub.1 structural element or which preferably start out directly from a C.sub.3 structural element, such as in particular acrolein. Where acrolein is used, as for example in the process according to DE-OS 39 26 136 (U.S. Pat. No. 5,015,789, incorporated by reference) or German patent applications P 40 38 192.7 (U.S. Pat. No. 5,171,898, incorporated by reference), P 41 39 981.6 and P 41 38 982.4 (U.S. patent application Ser. No. 07/981,324 filed on Nov. 24, 1992, now U.S. Pat. No. 5,276,201, incorporated by reference), it is hydrated in aqueous phase in the presence of an acidic hydration catalyst to form 3-hydroxypropionaldehyde (HPA).
The aqueous reaction mixture formed in the hydration of acrolein, which contains unreacted acrolein and secondary hydration products, such as 4-oxaheptanedial, and optionally dissolved hydration catalysts in addition to water and 3-hydroxypropionaldehyde, is hydrogenated in the presence of hydrogenation catalysts after removal of the unreacted acrolein and, optionally, part of the water. 1,3-Propanediol is recovered from the reaction mixture thus obtained by distillation and/or extraction based methods known to those skilled in the art.
According to U.S. Pat. No. 2,434,110 (incorporated by reference), catalysts containing one or more hydrogenation-active metals, such as Fe, Co, Ni, Cu, Ag, Mo, W, V, Cr, Rh, Pd, Os, Ir, Ru, or Pt, are suitable for the hydrogenation of 3-hydroxypropionaldehyde to 1,3-propanediol. As described in DE-OS 39 26 136, the catalyst may be present in suspended form per se or fixed to a support or may be a part of fixed-bed catalysts. Homogeneous catalysts may also be used. Such hydrogenation catalysts are well known in the art.
Preferred hydrogenation catalysts which provide for a substantially quantitative conversion of the 3-hydroxypropionaldehyde to 1,3-propanediol are (a) Raney nickel which may be doped with other catalytically active metals, (b) platinum-coated supported catalysts based on metal oxides or active carbon, such as in particular supported catalysts based on titanium dioxide containing 0.1 to 5.0% by weight, based on the support, of Pt in fine-particle form according to German patent application P 41 32 663.6, or (c) nickel-coated oxide- or silicate-containing supported catalysts, such as in particular those based on Ni/Al.sub.2 O.sub.3 /SiO.sub.2. A high volume/time yield is obtained in the hydrogenation reaction when the aqueous reaction mixture to be hydrogenated has an HPA content of 2 to 80% by weight, preferably 5 to 30% by weight and, more preferably, 5 to 20% by weight, and a pH value of 2.5 to 6.5; the hydrogenation temperature is in the range from 30.degree. to 180.degree. C. and the hydrogenation reaction is carried out under hydrogen pressures of 5 to 300 bar.
In the production of 1,3-propanediol on an industrial scale by hydrogenation of 3-hydroxypropionaldehyde in aqueous phase, it has been found that, even under the optimal reaction conditions mentioned above with an HPA conversion of substantially 100% and a selectivity approaching 100%, and despite careful working up of the reaction mixture by distillation and purification of the 1,3-propanediol by distillation, the 1,3-propanediol still has a residual carbonyl content, expressed as propionaldehyde, of a few thousand ppm. This high carbonyl content is extremely troublesome in the production of poly(1,3-propylene glycol terephthalate) and in the application of this polymer for the production of fibers by melt spinning because it leads both to odor emissions and to discoloration of the fibers.